Disposal of spent toners has long been a major problem for users of electrostatic printers and plotters. Today, ever increasing environmental awareness together with proliferating governmental regulations relating to chemical disposal are beginning to threaten use of liquid toners altogether. Despite the fact that the materials used for modern liquid toners are relatively harmless, the costs associated with proper disposal and documentation are threatening to render the technology non-competitive in the marketplace.
A method already exists, in principle, for eliminating the need for liquid toner disposal. This is described in U.S. Pat. Nos. 4,799,452; 4,895,103 and 4,923,581, all to G. F. Day. In this method the toner itself is eliminated except for a transitory existence just at the moment of toning. Concentrated "ink" of each color is stored in a small tank and injected into and mixed with a continuous stream of clear carrier fluid. The resulting toner stream is passed through the toner applicator and then immediately decomposed back into concentrate and clear fluid. This is done electrophoretically with a "separator", described in the patents. To stop the toning process the injection of the selected concentrate is simply terminated.
It would seem that this recycling concept might provide a liquid toning technology free of disposal problems since a large volume of contaminated or spent toner would never exist. However, the basic cause of disposal is not eliminated. Eventually the contents of the concentrate tanks would have to be discarded due to contamination. This is because the contaminants are re-mixed with the carrier fluid along with the colored toner particles and never removed from the system. The quantity of liquid to be thrown away would, to be sure, be smaller, but some fluid disposal problems would still remain. Also, the recycling architecture is relatively complex. It requires precise metering and mixing of two fluid streams and, worse, high speed separation of the toner into its components as it flows out of its applicator. With the high flows which are characteristic of full-width toner applicators, the separation apparatus must be quite large and, therefore, costly.
In order to electrophoretically separate a toner stream into its components the fluid is passed between two closely spaced, parallel electrodes while a high voltage is imposed across the gap. All of the fluid must be exposed to the full electric field and this means the flow must normally be confined to the gap region with some kind of fluid seals along the lateral edges of the separation zone. One of the electrodes must also be moving so that the accumulating sludge can be scraped off and sent to the appropriate concentrate tank. The seals which confine the fluid flow within the gap present numerous technical difficulties. They must support the high voltage and this high voltage appears across the exposed surface of the seal which joins the two electrodes. Surface electrical breakdown keeps the applied voltage lower than would otherwise be desired and this, in turn, causes the separator to be yet larger in order to assure total separation of the solids from the clear fluid. Also, four such large separators are normally required for a four color system.
An object of the invention was to devise a more effective method of providing disposal-free liquid toner than prior art recycling methods.